1. Field of the Invention
This invention relates to a process for reducing potential subterranean formation damage in pore spaces when clear completion fluids are used.
2. Description of the Art
Aqueous solutions of alkali and alkaline earth metals and mixtures thereof, are used as drilling, completion, packer and perforating media in oil and gas wells. The advent of the so-called solids free brines in recent years has greatly improved the completion and workover activities by eliminating formation plugging and solids settling problems previously encountered when water base and oil base drilling fluids were used. Clear brines are generally considered to be useful fluids for well servicing or completion of oil and gas wells because they are (1) free of solids, (2) inhibit swelling ard dispersion of clays in porous media and (3) do not change the wetability of subterranean formations. Despite these positive attributes, it has been shown that clear high density brines may cause damage to subterranean formations.
The possibility and degree of formation damage depends upon four factors. The four factors are (1) the chemical nature and density of the brines, (2) the physical and chemical nature of clays lining the pores, (3) the average formation pore pressure and (4) the compatibility of fluids present in the formation with the completion brine.
Depending upon their chemical make up, clear, high density brines interact with subterranean formations containing carbon dioxide. If the brine contains calcium, the reaction will result in carbonate precipitates, which may block pores in the subterranean formation. To date, the only solution to the formation of calcium precipitates has been the avoidance of the use of calcium brines when carbon dioxide is present.
Corley, et al., have addressed the problem of damage by clear brines in "Clear Fluids: They're Not Always Non-Damaging", World Oil, November 1984, p. 66-69. To minimize formation damage, the article suggests using a solids laden polymer-carbonate fluid.
Morgenthaler, in "Formation Damage Tests of High-Density Brine Completion Fluids", Society of Petroleum Engineers Production Engineering November 1986, p. 432-36 notes that formation damage occurs when trines having a density of 13.4-14.9 lbm/gal are used. According to Morgenthaler, highly concentrated brines containing calcium broxide or calcium chloride cause damage which he postulates may be due to the precipitation of an acid soluble calcium salt. The author recommends using zinc bromide to prevent formation impairment.
Calcium free systems such as sodium bromide, sodium chloride or zinc bromide solutions have been used to avoid subterranean formation damage. Although zinc carbonates are also insoluable, zinc carbonates do not usually form. The principle disadvantage of these systems is either the inability to achieve the desired fluid density or the unfavorable economics associated with these special fluids.
Accordingly, a primary objective of the present invention is to design a process to prevent subterranean damage when clear, high density brines are used in oil and gas well applications.
Another object of the invention is to inhibit the interaction of clear high density brines with the subterranean formation matrix in order to prevent formation damage.
Yet another object of the invention is to prevent the formation of precipitates due to the reaction between the clear high density brines and substances found in the subterranean formation.